Operation control device and operation control method for multi-cylinder internal combustion engine

ABSTRACT

When the cut-off cylinder operation execution condition of a multi-cylinder internal combustion engine is fulfilled, the temperature of the catalytic converter provided in the exhaust passage linked to the cylinders that are made inoperative during the cut-off cylinder operation is estimated, and when this temperature is less than the predetermined preliminary heating requiring temperature, a catalyst preliminary heating operation is executed by which the ignition timing of the cylinders is delayed and the temperature of the catalytic converter is raised. Then, the cut-off cylinder operation is started after the temperature of the catalytic converter becomes equal to or higher than the preliminary heating requiring temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an operation control device and an operation control method for a multi-cylinder internal combustion engine. More specifically, the invention relates to the improvement designed to extend the continuation time of a cut-off cylinder operation in a multi-cylinder internal combustion engine in which the cut-off cylinder operation can be executed by which some of the cylinders are made inoperative in response to a load of the internal combustion engine.

2. Description of the Related Art

A multi-cylinder engine is available in which a cut-off cylinder operation can be executed in which some of the cylinders are made inoperative and fuel consumption ratio is improved when no load is applied to the engine, as disclosed for example, in Japanese Patent Application Publication No. 2005-351134 (JP-A-2005-351134), Japanese Patent Application Publication No. 2001-227369 (JP-A-2001-227369), and Japanese Patent Application Publication No. 2001-182601 (JP-A-2001-182601).

For example, in a state with an extra output, as during idling operation of an engine, a load applied to each cylinder is small. Therefore, the intake-exhaust loss increases and combustion efficiency can drop. For this reason, a cut-off cylinder operation can be performed in which fuel supply to some cylinders (for example, cylinders of one bank in a V-type engine) is stopped and these cylinders are caused to rest (made inoperative), a load applied to operative cylinders (cylinders of the other bank) to which fuel is supplied is increased, and the combustion efficiency is increased. As a result, the fuel consumption ratio can be improved and the amount of consumed fuel can be reduced.

JP-A-2005-351134 discloses a configuration in which a transition is made from the above-described cut-off cylinder operation to the full-cylinder operation (operation in which fuel is supplied to all the cylinders), when the temperature of the catalyst provided in the exhaust system connected to the cylinder (inoperative cylinder) that has hereintofore been in an inoperative state is lower than the activation temperature, the ignition timing in the combustion stroke of this cylinder is delayed. As a result, the catalyst temperature is rapidly increased and exhaust emission is improved.

JP-A-2001-227369 discloses a configuration in which the cut-off cylinder operation is prohibited in a case where the temperature of the catalyst provided in the exhaust system connected to the inoperative cylinder is lower than a predetermined temperature (a temperature at which the catalyst is assumed to be in the actually active state). In other words, the transition to the full-cylinder operation is made as the catalyst temperature decreases, thereby preventing the degradation of exhaust emission during the restoration of the full-cylinder operation that is caused by the decrease in the catalyst temperature to a level equal to or below the activation temperature.

JP-A-2001-182601 discloses a configuration in which the ignition is delayed and exhaust gas temperature is raised when the catalyst rapid warm-up control is executed and the cut-off cylinder operation is performed at this time by stopping the injection of fuel in some cylinders.

However, the following problem can arise in an engine in which the cut-off cylinder operation can be executed when the engine is so configured that the catalyst of the exhaust system that is connected to a cylinder (inoperative cylinder) that is in an inoperative state during the cut-off cylinder operation (this catalyst will be referred to hereinbelow as an inoperative cylinder catalyst) and the catalyst of the exhaust system connected to a cylinder (operative cylinder) that continues operating (this catalyst will be referred to hereinbelow as an operative cylinder catalyst) are independent from each other, as disclosed in JP-A-2005-351134 and JP-A-2001-227369.

In other words, when the cut-off cylinder operation execution condition is satisfied, for example, because the engine is in an idling state, even if the temperature of the inoperative cylinder catalyst is equal to or higher than the lower limit of the activation temperature (for example, 450° C.), where the catalyst temperature is not sufficiently high (at least 50° C. higher than the lower limit of the activation temperature of the catalyst) the catalyst temperature drops to the vicinity of the lower limit of the activation temperature within a short period after the cut-off cylinder operation has been started and the full-cylinder operation has to be restored.

In particular, in an engine in which intake valves and exhaust valves of the inoperative cylinder during the cut-off cylinder operation are opened and closed in the same manner as during the full-cylinder operation, the air (air of about the same temperature as the external air) flows to the inoperative cylinder catalyst during the cut-off cylinder operation. As a result, the decrease in the catalyst temperature per unit time is easily increased and the above-described problem is aggravated.

In such cases the continuation time of the cut-off cylinder operation becomes extremely short and merits of the engine system in which the cut-off cylinder operation can be executed are difficult to use to a full extent. As a result, it is possible that the effect of improving the fuel consumption ratio and the effect of reducing the fuel consumption amount will not be fully demonstrated.

This problem can similarly occur in the configuration described in JP-A-2005-351134, JP-A-2001-227369, and JP-A-2001-182601. In other words, in the configuration described in JP-A-2005-351134, the catalyst temperature is raised by delaying the ignition timing after a transition has been made from the cut-off cylinder operation to the full-cylinder operation, and such a technique cannot extend the continuation period of the cut-off cylinder operation by extending the period in which the temperature of the inoperative cylinder catalyst in the cut-off cylinder operation is maintained at the activation temperature.

In the configuration disclosed in JP-A-2001-227369, the cut-off cylinder operation is prohibited when the catalyst temperature becomes lower than the predetermined temperature, and this technique also fails to extend the interval in which the temperature of the inoperative cylinder catalyst in the cut-off cylinder operation is maintained at the activation temperature.

In the configuration of the exhaust system disclosed in JP-A-2001-182601, all cylinders are linked to the same catalyst (therefore, it is not a configuration in which the inoperative cylinder catalyst and operative cylinder catalyst are independent). As a result, the above-described problem of the temperature of some of the catalysts decreasing in the execution of the cut-off cylinder operation is not encountered. In other words, this is not the exhaust system structure that is the object of the invention.

SUMMARY OF THE INVENTION

The invention relates to a multi-cylinder internal combustion engine in which the cut-off cylinder operation can be executed and provides an operation control device and operation control method for a multi-cylinder internal combustion engine that can maintain a high temperature of the inoperative cylinder catalyst during the cut-off cylinder operation and ensure a long continuation time of the cut-off cylinder operation.

The problem resolution principle on which the invention is based is in executing control by which the catalyst temperature of the exhaust system linked to a cylinder (cut-off scheduled cylinder) that will be in an inoperative state during the cut-off cylinder operation is set high in advance and, then the cut-off cylinder operation is started, As a result, the interval in which the catalyst temperature decreases to the vicinity of the lower limit of the activation temperature during the cut-off cylinder operation can be extended and the continuation time of the cut-off cylinder operation can be extended.

The first aspect of the invention relates to an operation control device for a multi-cylinder internal combustion engine which is provided with a first catalyst provided in a first exhaust passage linked to at least one cylinder from among a plurality of cylinders and a second catalyst provided in a second exhaust passage linked to other cylinders, and in which a cut-off cylinder operation that makes the at least one cylinder inoperative can be executed following the fulfillment of a predetermined cut-off cylinder operation execution condition. The operation control device for a multi-cylinder internal combustion engine includes catalyst temperature recognition means and catalyst preliminary heating operation execution means. The catalyst temperature recognition means detects or estimates a temperature of the first catalyst. The catalyst preliminary heating operation execution means executes a catalyst preliminary heating operation for increasing a temperature of the first catalyst to a temperature equal to or higher than a preliminary heating requiring temperature before the cut-off cylinder operation is executed in a case where the temperature of the first catalyst that has been detected or estimated by the catalyst temperature recognition means is less than the predetermined preliminary heating requiring temperature when the cut-off cylinder operation execution condition is fulfilled.

The expression “at least one cylinder” that becomes inoperative during the cut-off cylinder operation means not only a single cylinder, but also a plurality of cylinders (for examples, the cylinders of one bank in a V-type engine). Further, catalysts not only of two types, namely, the first catalyst and the second catalyst, but of three or more types may be also disposed independently from each other as the catalysts that are disposed independently by providing them in mutually different exhaust passages.

According to the above-described specific feature, the temperature of the first catalyst is detected or estimated by the catalyst temperature recognition means when the cut-off cylinder operation execution condition is fulfilled in the process of executing the full-cylinder operation. When the temperature of the first catalyst is less than the predetermined preliminary heating requiring temperature, first, a catalyst preliminary heating operation is executed to raise the temperature of the first catalyst to a temperature equal to or higher than the preliminary heating requiring temperature, instead of starting the cut-off cylinder operation immediately after the cut-off cylinder operation execution conditions has been fulfilled. The cut-off cylinder operation is then started after the temperature of the first catalyst has been sufficiently raised to a temperature equal to or higher than the preliminary heating requiring temperature by the execution of the catalyst preliminary heating operation. In other words, because the cut-off cylinder operation is started in a state with a comparatively high temperature of the first catalyst, even if the temperature of the first catalyst decreases gradually during the cut-off cylinder operation, the time required for this temperature to decrease to a temperature at which the restoration of the full-cylinder operation is necessary (for example, the lower limit of the activation temperature) can be extended. As a result, the continuation time of the cut-off cylinder operation can be extended, merits of the engine system in which the cut-off cylinder operation can be executed can be used to a full extent, and the effect of improving the fuel consumption ratio and the effect of reducing the fuel consumption amount can be fully demonstrated.

The catalyst preliminary heating operation execution means may be configured to execute, prior to executing the cut-off cylinder operation, a catalyst preliminary heating operation of shifting an ignition timing of a spark plug of the at least one cylinder that becomes inoperative during the cut-off cylinder operation to a delay side, or a catalyst preliminary heating operation of un-igniting the spark plug.

When the catalyst preliminary heating operation is executed that delays the ignition timing of the spark plug, the combustion start timing in the combustion process of the aforementioned at least one cylinder (a cylinder that is inoperative when a transition is made to the cut-off cylinder operation) is also delayed, the combustion in these cylinders is slowed down, and a state is assumed in which part of the air-fuel mixture burns in the exhaust passage (first exhaust passage). As a result, the temperature of the first catalyst can be rapidly raised by raising significantly the gas temperature inside the exhaust system. Therefore, the temperature of the first catalyst can be raised sufficiently and within a short time to a temperature that is equal to or higher than the preliminary heating requiring temperature and the fuel supplied within the catalyst preliminary heating operation period can be effectively caused to participate in the temperature increase of the first catalyst. As a result, the amount of fuel used for the catalyst preliminary heating operation can be made comparatively small and a significant increase in fuel consumption amount can be prevented.

In a case where the catalyst preliminary heating operation is executed that un-ignites, the spark plug, practically the entire air-fuel mixture contained in the cylinder enters as an unburned gas the exhaust passage (first exhaust passage), thermal energy of the first catalyst is received, and the mixture is burned (oxidation reaction). In this case, the temperature of the first catalyst also can be raised sufficiently and within a short time to a temperature that is equal to or higher than the preliminary heating requiring temperature. Further, in this case, almost the entire amount of fuel supplied within the catalyst preliminary heating operation period can be caused to participate in the temperature increase of the first catalyst. As a result, the amount of fuel used for the catalyst preliminary heating operation can be reduced to a necessary minimum. Thus, where the catalyst preliminary heating operation is executed that un-ignites the spark plug, certain thermal energy has to be present inside the first catalyst. Therefore, this operation may be performed after the temperature of the first catalyst has been detected or estimated and the presence of thermal energy sufficient for burning the air-fuel mixture inside the first catalyst has been confirmed.

Further, the catalyst preliminary heating operation may delay an ignition timing of the spark plugs of the at least one cylinder that becomes inoperative during the cut-off cylinder operation prior to executing the cut-off cylinder operation and gradually increase the delay amount of the ignition timing, thereby gradually decreasing the amount of kinetic energy participating in torque generation in the internal combustion engine and gradually increasing the amount of thermal energy participating in the catalyst preliminary heating, from among the energy generated by combustion of an air-fuel mixture.

With such a configuration, where the cut-off cylinder operation execution condition is fulfilled and the catalyst preliminary heating operation is started, in the initial period thereof, the delay amount of the ignition timing of the spark plug is comparatively small, an a major portion of energy generated by the combustion of the air-fuel mixture becomes kinetic energy participating in torque generation in the internal combustion engine, whereas part of the energy is used as thermal energy participating in the catalyst preliminary heating (heating of the first catalyst). Then, as the delay amount of the ignition timing of the spark plugs gradually increases, the amount of kinetic energy participating in torque generation in the internal combustion engine, from among the energy generated by the combustion of the air-fuel mixture, gradually decreases, whereas the amount of thermal energy participating in the catalyst preliminary heating gradually increases. Where the temperature of the first catalyst becomes equal to or higher than the preliminary heating requiring temperature, the catalyst preliminary heating operation is stopped and the cut-off cylinder operation is started. Thus, when switching is conducted from the full-cylinder operation to the cut-off cylinder operation, the torque of the internal combustion engine gradually decreases during the catalyst preliminary heating operation. Therefore, abrupt torque fluctuations during such operation mode switching are prevented and the occurrence of vibrations (shocks) during the operation mode switching is practically eliminated. In other words, switching from the full-cylinder operation to the cut-off cylinder operation can be so conducted that the vehicle occupants will not be aware of the operation mode switching and drivability can be greatly improved. Thus, with the present problem resolution means, it is possible to realize a novel operation of switching from the full-cylinder operation to the cut-off cylinder operation that causes practically no vibrations during operation switching and also attain the effect of improving the fuel consumption ratio and the effect of reducing the fuel consumption ratio by extending the continuation time of the cut-off cylinder operation.

Further, the catalyst preliminary heating operation execution means may also shift a fuel supply timing of the at least one cylinder that becomes inoperative during the cut-off cylinder operation to a delay side prior to executing the cut-off cylinder operation.

In this case, the air-fuel mixture is also burned in the first exhaust passage or the first catalyst. Therefore, the temperature of the first catalyst can be sufficiently raised within a short time to a temperature equal to or higher than the preliminary heating requiring temperature.

In a case where the temperature of the first catalyst has decreased to a predetermined full-cylinder operation restoration temperature during the cut-off cylinder operation, switching may be made from the cut-off cylinder operation to the full-cylinder operation. Further, the full-cylinder operation restoration temperature may be set to a temperature that is higher than a lower limit of the activation temperature of the first catalyst at which the air-fuel mixture can burn inside the first catalytic converter and to a temperature that is lower than the preliminary heating requiring temperature (lower limit of the activation temperature of the first catalyst)<(full-cylinder operation restoration temperature)<(preliminary heating requiring temperature).

Where the temperatures are so set (the preliminary heating requiring temperature is set higher than the full-cylinder operation restoration temperature, and the full-cylinder operation restoration temperature is set higher than the lower limit of the activation temperature of the first catalyst), it is possible both to extend the time required for the catalyst temperature to decrease to the vicinity of the lower limit of the activation temperature during the cut-off cylinder operation and to prevent the degradation of exhaust emission when switching is conducted from the cut-off cylinder operation to the full-cylinder operation.

The second aspect of the invention relates to an operation control method for a multi-cylinder internal combustion engine which is provided with a first catalyst provided in a first exhaust passage linked to at least one cylinder from among a plurality of cylinders and a second catalyst provided in a second exhaust passage linked to other cylinders, and in which a cut-off cylinder operation that makes the at least one cylinder inoperative can be executed. The operation control method includes determining whether the predetermined cut-off cylinder operation execution condition is fulfilled; detecting or estimating a temperature of the first catalyst; and executing a catalyst preliminary heating operation for increasing a temperature of the first catalyst to a temperature equal to or higher than a preliminary heating requiring temperature before the cut-off cylinder operation is executed in a case where the temperature of the first catalyst that has been detected or estimated is less than the predetermined preliminary heating requiring temperature when the cut-off cylinder operation execution condition is fulfilled.

According to the second aspect of the invention, the continuation time of the cut-off cylinder operation can be extended, merits of the engine system in which the cut-off cylinder operation can be executed can be used to a full extent, and the effect of improving the fuel consumption ratio and the effect of reducing the fuel consumption amount can be fully demonstrated.

In accordance with the invention, when the cut-off cylinder operation is started, the control is executed by which the catalyst temperature of the exhaust system linked to the cylinders that will be made inoperative in the cut-off cylinder operation is set high in advance, and then the cut-off cylinder operation is started. As a result, the time required for the catalyst temperature to decrease to the vicinity of the lower limit of the activation temperature during the cut-off cylinder operation can be extended. As a result, the continuation time of the cut-off cylinder operation can be extended, and the effect of improving the fuel consumption ratio and the effect of reducing the fuel consumption amount can be fully demonstrated.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein:

FIG. 1 shows a schematic configuration of the internal structure of a V-type engine of the present embodiment that is viewed from the direction along the central axis of a crankshaft;

FIG. 2 is a system configuration diagram illustrating schematically the engine, an intake-exhaust system, and a control system;

FIG. 3 is a block diagram illustrating the control system of the engine; and

FIG. 4 is a flowchart illustrating the operation sequence of operation switching control.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of the invention will be described below with reference to the appended drawings. In the present embodiment, a case is explained in which the invention is applied to a vehicle that has installed therein a V-type six-cylinder gasoline engine as an internal combustion engine.

Before explaining the control performed during switching to the cut-off cylinder operation, which is the control representing a specific feature of the invention, the general configuration of the engine and a control block will be explained.

FIG. 1 shows a schematic configuration of the internal structure of the V-type engine E of the present embodiment that is viewed from the direction along the central axis of a crankshaft C. FIG. 2 is a system configuration diagram illustrating schematically the engine E, an intake-exhaust system, and a control system.

As shown in these figures, the V-type engine E has a pair of banks 2L, 2R protruding in a V-like shape in the upper portion of a cylinder block 1. The banks 2L, 2R are provided with cylinder heads 3L, 3R disposed in the upper end portion of the cylinder block 1 and head covers 4L, 4R attached to the upper ends of the cylinder heads. A plurality of cylinders 5L, 5R (for example, three cylinders in each bank 2L, 2R) are installed at a predetermined insertion angle (for example, 90°) in the cylinder block 1, and pistons 51L, 51R are accommodated inside the cylinders 5L, 5R so that the cylinders can reciprocate therein. The pistons 51L, 51R are coupled via connecting rods 52L, 52R to the crankshaft C so that power can be transmitted thereto. Further, a crankcase 6 is mounted below the cylinder block 1, and a space from the lower portion inside the cylinder block 1 to the inside of the crankcase 6 serves as a crank chamber 61. An oil pan 62 that serves as an oil reservoir is provided further below the crankcase 6.

Intake valves 32L, 32R for opening and closing intake ports 31L, 31R and exhaust vales 34L, 34R for opening and closing the exhaust ports 33L, 33R are incorporated in the cylinder heads 3L, 3R, respectively, and the opening and closing operation of the valves 32L, 32R, 34L, 34R is performed by the rotation of camshafts 35L, 35R, 36L, 36R disposed in cam chambers 41L, 41R formed between the cylinder heads 3L, 3R and head covers 4L, 4R.

The cylinder heads 3L, 3R of the engine E of the present embodiment have a divided structure. More specifically, the cylinder heads 3L, 3R are configured by a main cylinder head body 37L, 37R mounted on the upper surface of the cylinder block 1 and camshaft housings 38L, 38R attached at the upper side of the main cylinder head bodies 37L, 37R.

Intake manifolds 7L, 7R that correspond to the banks 2L, 2R are disposed in the upper portion on the inside (side between the banks) of the banks 2L, 2R, and the downstream ends of the intake manifolds 7L, 7R communicate with the intake ports 31L, 31R. Further, the intake manifolds 7L, 7R communicate with a surge tank 71 (see FIG. 2) that is shared by the banks and an intake pipe 73 provided with a throttle valve 72. An air cleaner 74 is provided upstream of the intake pipe 73. As a result, the air introduced from the air cleaner 74 into the intake pipe 73 is introduced into intake manifolds 7L, 7R via the surge tank 71.

Injectors 75L, 75R are provided in the intake ports 31L, 31R of the cylinder heads 3L, 3R, respectively, and when fuel is injected from the injectors 75L, 75R, an air-fuel mixture is obtained in which the air introduced in the intake manifolds 7L, 7R is mixed with the fuel injected from the injectors 75L, 75R, and as the intake valves 32L, 32R are opened, the air-fuel mixture is introduced into combustion chambers 76L, 76R.

Spark plugs 77L, 77R are disposed in the top portions of the combustion chambers 76L, 76R. In the combustion chambers 76L, 76R, the combustion pressure of the air-fuel mixture that is produced by the ignition of the spark plugs 77L, 77R is transmitted to pistons 51L, 51R causing reciprocating movement of the piston 51L, 51R. The reciprocating movement of the pistons 51L, 51R is transmitted to the crankshaft C via connecting rods 52L, 52R, converted into rotational movement, and taken out as an output of the engine E. The cam shafts 35L, 35R, 36L, 36R are rotationally driven by the power outputted from the crankshaft C that is transmitted by a timing chain, and this rotation opens and closes the valves 32L, 32R, 34L, 34R.

The air-fuel mixture after combustion becomes an exhaust gas that is released to the exhaust manifolds 8L, 8R when the exhaust valves 34L, 34R are opened. Exhaust pipes 81L, 81R are connected to the exhaust manifolds 8L, 8R, and catalyst converters 82L, 82R containing a three-way catalyst are attached to the exhaust pipes 81L, 81R. When the exhaust gas passes through the catalytic converters 82L, 82R, hydrogen carbide (HC), carbon monoxide (CO), and nitrogen oxide component (NOx) contained in the exhaust gas are purified. Further, the downstream end sides of the exhaust pipes 81L, 81R are merged and connected to a muffler 83.

The operation state of the above-described engine E is controlled by an engine Electronic Control Unit (ECU) 9. As shown in FIG. 3, the engine ECU 9 is provided with a Central Processing Unit (CPU) 91, a Read Only Memory (ROM) 92, a Random Access Memory (RAM) 93, and a backup RAM 94.

The ROM 92 stores control programs and maps that are referred to when the control programs are executed. The CPU 91 executes computations on the basis of the control programs and maps stored in the ROM 92.

The RAM 93 is a memory that temporarily stores the computation results obtained in the CPU 91 and also data inputted from sensors. The backup RAM 94 is a nonvolatile memory that stores data that have to be saved when the engine E is stopped. These ROM 92, CPU 91, RAM 93, and backup RAM 94 are connected to each other via a bus 97 and also connected to an external input circuit 95 and external output circuit 96.

A water temperature sensor 101, an air flowmeter 102, an intake temperature sensor 103, an A/F sensor 104 a, an O₂ sensor 104 b, a throttle position sensor 105, a crank angle sensor 106, a cam angle sensor 107, a knocking sensor 108, an intake pressure sensor 109, and an accelerator depression amount sensor 110 are connected to the external input circuit 95. The injectors 75L, 75R, an igniter 111, and a throttle motor 72 a that drives the throttle valve 72 are connected to the external output circuit 96.

The water temperature sensor 101 detects the temperature of cooling water that flows inside a water jacket 11 that is formed in the cylinder block 1 and sends a cooling water temperature signal to the engine ECU 9.

The air flowmeter 102 detects the intake air amount and sends an intake air amount signal to the engine ECU 9.

The intake temperature sensor 103 is provided downstream of the air cleaner 74, detects an intake air temperature, and sends an intake temperature signal to the engine ECU 9.

The A/F sensor 104 a is provided upstream of the catalyst converters 82L, 82R. For example, an oxygen concentration sensor of a limited current system can be used therefor. The A/F sensor 104 a generates an output voltage corresponding to an air-fuel ratio over a wide air-fuel ratio range and sends a voltage signal to the engine ECU 9.

The O₂ sensor 104 b is provided downstream of the catalyst converters 82L, 82R. For example, an oxygen concentration sensor of an electromotive force system (density battery system) can be used. The O₂ sensor 104 b determines whether the air-fuel ratio in the exhaust gas is a stoichiometric air-fuel ratio and sends a detection signal to the engine ECU 9.

The throttle position sensor 105 detects an opening degree of the throttle valve 72 and sends a throttle opening degree detection signal to the engine ECU 9.

The crank angle sensor 106 is disposed in the vicinity of the crankshaft C and detects a rotation angle (crank angle CA) of the crankshaft C and a revolution speed (engine revolution speed NE). More specifically, the crank angle sensor 106 outputs a pulse signal for each predetermined crank angle (for example, 30°). An example of a method for detecting the crank angle with the crank angle sensor 106 is described below. Thus, an outer tooth is formed for every 30° on the outer circumferential surface of a rotor (NE rotor) 106 a that is rotationally integrated with the crankshaft C and the crank angle sensor 106 configured by an electromagnetic pickup is disposed opposite the outer tooth. When the outer tooth passes in the vicinity of the crank angle sensor 106, following the rotation of the crankshaft C, the crank angle sensor 106 generates an output pulse. In some cases the NE rotor 106 a is used in which the outer tooth is formed for every 10° on the outer circumferential surface. In this case, an output pulse is generated from every 30° CA by frequency division in the engine ECU 9.

The cam angle sensor 107 is provided in the vicinity of the intake cam shafts 35L, 35R and used as a cylinder determination sensor by outputting a pulse signal correspondingly to a Top Dead Center (TDC), for example, of the first cylinder. In other words, the cam angle sensor 107 outputs a pulse signal for each revolution of the intake cam shafts 35L, 35R. An example of a method for detecting the cam angle with the cam angle sensor 107 is described below. Thus, an outer tooth is formed in one position on the outer circumferential surface of a rotor that is rotationally integrated with the intake cam shaft 35L, 35R, and the cam angle sensor 107 configured by an electromagnetic pickup is disposed opposite the outer tooth. When the outer tooth passes in the vicinity of the cam angle sensor 107, following the rotation of the intake cam shaft 35L, 35R, the cam angle sensor 107 generates an output pulse. The above rotor rotates at half the speed of the crankshaft C. Hence the crankshaft C generates an output pulse every time the crankshaft rotates 720°. In other words, the configuration is provided where an output pulse is generated every time a certain cylinder exerts the same stroke (for example, when the first cylinder is at the compressive top dead).

The knocking sensor 108 is provided for each bank 2L, 2R. It is a vibration-type sensor that detects engine vibrations that are transmitted to the cylinder block 1 with a piezoelectric system (piezo element) or an electromagnetic system (a magnet and a coil). An output signal corresponding to the amplitude of vibrations of the cylinder block 1 is sent to the engine ECU 9.

The intake pressure sensor 109 is mounted on the surge tank 71, detect a pressure inside the intake pipe 73 (internal pressure of intake pipe), and sends an intake pressure signal to the engine ECU 9.

The accelerator depression amount sensor 110 outputs a detection signal corresponding to the depression amount of the accelerator pedal (accelerator depression amount). The operation speed of the accelerator can be recognized by recognizing the variation amount of the accelerator depression amount per unit time.

The engine ECU 9 then executes various types of control of the engine E, including the ignition timing control, by controlling various units such as the igniter 111, injector 75L, 75R, and throttle motor 72 a, on the basis of output signals of the sensors 101 to 110.

As an example, when knocking is detected by the knock sensors 108, 108, while conducting the advance correction of the ignition timing so as to bring the ignition timing to the vicinity of Minimum Spark Advance for Best Torque (MBT), a control is conducted to perform the delay correction of the ignition timing and eliminate the knocking as the basic control of ignition timing of the spark plugs 77L, 77R determined by the igniters 111, 111.

Further, for the control of the injectors 75L, 75R, the target air-fuel ratio is calculated on the basis of engine load or engine revolution speed and a control of fuel ignition amount (control of opening time of injectors 75L, 75R) is conducted so as to obtain the target air-fuel ratio on the basis of intake air amount detected by the air flowmeter 102. In this case, oxygen concentration in the exhaust gas is calculated on the basis of outputs of the A/F sensor 104 a and O₂ sensor 104 b and an air-fuel ratio feedback control is conducted by which the fuel injection amount determined by the injectors 75L, 75R is controlled so as to match the actual air-fuel ratio obtained from the calculated oxygen concentration with the target air-fuel ratio (for example, stoichiometric air-fuel ratio).

As the drive control of the throttle motor 72 a, the drive amount of the throttle motor 72 a is controlled so as to obtain an opening degree of the throttle valve 72 ensuring an intake air amount necessary to obtain the required engine output, the control being conducted based on the depression amount of accelerator pedal operated by the driver.

The engine ECU 9 also executes the below-described cut-out cylinder operation control. The cut-out cylinder operation will be described below.

In the V-type engine E of the present embodiment, the cut-out cylinder operation can be conducted to stop the operation of a group of cylinders (three cylinders in the present embodiment) that belong to one bank (for example, the left bank 2L) from among the left bank 2L and the right bank 2R. In other words, in a state with extra power, for example during idling operation of the engine E, the load applied to each cylinder is small. Therefore, the intake-exhaust load increases and combustion efficiency is degraded. For this reason, in a no-load state or a low-load state, the cut-out cylinder operation is conducted by which the supply of fuel to cylinders of one bank is cut off and these cylinders are made inoperative. As a result, a load on the operative cylinders (cylinders of the other bank) to which the fuel is supplied is increased and the operation efficiency is raised, thereby improving the fuel consumption ratio.

As a specific example of the cut-out cylinder operation, it is determined whether the engine E is in a no-load state or low-load state, e.g. in an idling state, on the basis of the engine revolution speed calculated based on the output signal from the crank angle sensor 106 and the opening degree of the throttle valve 72 detected by the throttle position sensor 105, and when the engine is in the no-load state or low-load state, it is determined that the cut-out cylinder operation execution condition is fulfilled.

In the present embodiment, when the cut-out cylinder operation is executed, it is always the three cylinders of the left bank 2L that are made to be inoperative. The reason therefor can be explained as follows. Thus, because a configuration is used in which the evaporated fuel generated in the fuel tank is introduced in the intake manifold 7R of the right bank 2R (not shown in the figure) and this evaporated fuel has to be treated, the three cylinders of the right bank 2R are maintained in the operative state.

Such a cut-out cylinder operation mode is not limiting, and it is also possible that a bank that has been operative in the previous cut-out cylinder operation be made inoperative, whereas the bank that has been inoperative in the previous cut-out cylinder operation be made operative when a transition is made to the cut-out cylinder operation. In other words, the bank that is made inoperative is switched alternately each time the cut-out cylinder operation is started, thereby ensuring a uniform distribution of accumulated operation time between the cylinders and extending the service life of the engine E.

In the present embodiment, the opening and closing operations performed with respect to the intake valve 32L and exhaust valve 34L of the inoperative cylinder during the cut-out cylinder operation are similar to those performed during the full-cylinder operation. As a result, the engine E of the present embodiment can be constructed without the necessity of subjecting the conventional engine in which the cut-out cylinder operation is not executed to significant design modifications.

In the cut-out cylinder operation, the intake valve 32L and exhaust valve 34L of the inoperative cylinder may be fully closed. In this case, a pumping loss caused by the reciprocating movement of the piston 51L in the inoperative cylinder can be reduced and the efficiency of the engine E can be increased.

An operation control that is a specific feature of the present embodiment in which the operation is switched between the full-cylinder operation and the cut-out cylinder operation will be described below.

Thus, in a case where the aforementioned cut-out cylinder operation execution condition is fulfilled, a temperature of the catalytic converter 82L provided in the exhaust pipe (exhaust pipe constituting the first exhaust passage) 81L linked to the three cylinders of the left bank 2L, that is, the cylinders that are made inoperative, is estimated, and when the temperature is less than a predetermined preliminary heating requiring temperature, the catalyst preliminary heating operation is executed to raise the temperature of the catalytic converter 82L prior to executing the cut-out cylinder operation (the catalyst preliminary heating operation is executed by the catalyst preliminary heating operation execution means).

In the explanation below, the catalytic converter 82L provided in the exhaust pipe 81L linked to the three cylinders of the left bank 2L that are made inoperative during the cut-out cylinder operation will be called a first catalyst converter (a first catalyst) 82L, and the catalytic converter 82R provided in the exhaust pipe (exhaust pipe constituting the second exhaust passage) 81R linked to the three cylinders of the right bank 2R that continue operating even during the cut-out cylinder operation will be called a second catalyst converter (a second catalyst) 82R.

FIG. 4 is a flowchart illustrating the sequence of operation switching control by which the operation is switched between the full-cylinder operation and cut-out cylinder operation. The routine shown in FIG. 4 is executed with a predetermined period or for each predetermined angle revolution of the crankshaft C.

First, in step ST1, it is determined whether the cut-out cylinder operation execution condition is fulfilled. As mentioned above, examples of the cut-out cylinder operation execution condition include whether or not the engine E is in a no-load state or low-load state, such as idling operation, and also whether the temperature of cooling water detected by the water temperature sensor 101 is equal to or higher than the predetermined temperature (for example, 50°). In the present embodiment, the temperature condition of the first catalytic converter 82L is not included in the cut-out cylinder operation execution condition determined in this step ST1. In other words, it is determined YES in step ST1 when the other cut-out cylinder operation execution condition is fulfilled, regardless of the temperature of the first catalytic converter 82L.

In a case where the cut-out cylinder operation execution condition is not fulfilled, it is determined NO in step ST1, the processing flow advances to step ST7 and the full-cylinder operation is continued. Further, where it is determined NO in step ST1 during the cut-out cylinder operation execution, the full-cylinder operation is restored.

Where the cut-out cylinder operation execution condition is fulfilled, it is determined YES in step ST1 and the processing flow advances to step ST2. In step ST2, it is determined whether the present operation state of the engine E is the full-cylinder operation state. In other words, it is determined whether a state has been assumed in which the cut-out cylinder operation execution condition is fulfilled from the state in which the full-cylinder operation is conducted because the cut-out cylinder operation execution condition has not been heretofore fulfilled, or it is determined whether a transition has not yet been made to the cut-out cylinder operation in the course of executing the below-described catalyst preliminary heating operation and the full-cylinder operation is conducted.

In a case where the engine E is in the full-cylinder operation mode and it is determined YES in step ST2, the processing flow advances to step ST3, and it is determined whether the temperature of the first catalytic converter 82L is less than the predetermined preliminary heating requiring temperature (T1). In other words, it is determined whether the temperature of the first catalytic converter 82L will drop to the vicinity of the lower limit of the activation temperature within a very short time and will become necessary to switch rapidly to the full-cylinder operation mode if the transition is directly made to the cut-out cylinder operation.

The temperature about 150° C. higher than the lower limit of the activation temperature (for example, 450° C.) of the first catalytic converter 82L is set as the preliminary heating requiring temperature (T1). However, these values are not limiting.

The temperature of the first catalytic converter 82L is estimated based of the present engine operation state. More specifically, a catalyst temperature estimation map for estimating the temperature of the first catalytic converter 82L from the engine revolution speed and engine load (throttle valve opening degree and the like) is stored in the ROM 92, and the temperature of the first catalytic converter 82L is estimated (catalyst temperature estimation operation performed with the catalyst temperature recognition means) by substituting the present engine revolution speed and engine load into the catalyst temperature estimation map. Further, the temperature of the first catalytic converter 82L may be also estimated from the exhaust gas temperature. For example, exhaust gas temperature sensors can be provided upstream and downstream of the first catalytic converter 82L, and the temperature of the first catalytic converter 82L can be estimated based on the exhaust gas temperature detected by these exhaust gas temperature sensors. The temperature of the first catalytic converter 82L may be also directly detected by using appropriate means such as a thermistor (operation of detecting the catalyst temperature with the catalyst temperature recognition means).

Where the temperature of the first catalytic converter 82L is equal to or higher than the preliminary heating requiring temperature (it is determined NO in step ST3), the processing flow advances to step ST6 and the cut-off cylinder operation is executed under an assumption that a sufficient time can be ensured till the temperature of the first catalytic converter 82L decreases to the vicinity of the lower limit of the activation temperature even though a transition is directly made to the cut-off cylinder operation. In other words, fuel injection to the cylinders of the left bank 2L is stopped and the cylinders are made inoperative.

Where the temperature of the first catalytic converter 82L is lower than the preliminary heating requiring temperature and it is determined YES in step ST3, the processing flow advances to step ST4 and the catalyst preliminary heating operation is executed. A catalyst preliminary heating operation includes delaying the ignition timing of the spark plugs 77L provided in the cylinders of the left bank 2L, in a state in which the full-cylinder operation is continued, that is, fuel injection to these cylinders is continued.

More specifically, the delay amount of the ignition timing is gradually increased. For example, the ignition timing is delayed by 1° CA (1° of crank angle) for every 10 revolutions of the crankshaft C. However, this value is not limiting and can be determined in advance by a test or simulation.

As a result, the combustion start timing in the combustion process of these cylinders (cylinders that become inoperative when a transition is made to the cut-off cylinder operation: cut-off scheduled cylinders), the combustion is slowed down, and a state is assumed in which part of the air-fuel mixture is burned in the exhaust pipe 81L. Therefore, by greatly increasing the gas temperature in the exhaust system provided with the first catalytic converter 82L, it is possible to raise rapidly the temperature of the first catalytic converter 82L.

Further, where the delay amount of the ignition timing is gradually increased, the amount of kinetic energy participating in torque generation in the engine E, from among the energy generated by the combustion of the air-fuel mixture in the cylinders, is gradually decreased and the amount of thermal energy participating in preliminary heating of the first catalytic converter 82L is gradually increased. In other words, where the cut-off cylinder operation execution condition is fulfilled and the catalyst preliminary heating operation is started, in the initial period thereof, the delay amount of the ignition timing of the spark plug 77L is comparatively small, a major portion of energy generated by the combustion of the air-fuel mixture becomes kinetic energy participating in torque generation in the internal combustion engine E, whereas part of the energy is used as thermal energy participating in the preliminary heating of the first catalytic converter 82L. Then, as the delay amount of the ignition timing of the spark plug 77L gradually increases, the amount of kinetic energy participating in torque generation in the internal combustion engine E, from among the energy generated by the combustion of the air-fuel mixture, gradually decreases, whereas the amount of thermal energy participating in the preliminary heating of the first catalytic converter 82L gradually increases. Thus, in the catalyst preliminary heating operation, the torque of the engine E is gradually decreased, while the preliminary heating capacity of the first catalytic converter 82L is being gradually increased.

In this case, the maximum delay amount of the ignition timing is set such that the lower limit value of the torque of the engine E that is varied so as to decrease gradually (the lower limit value of the torque of the engine E during the catalyst preliminary heating operation) almost matches the torque of the engine E during the below-described transition from the full-cylinder operation to the cut-off cylinder operation (torque of the engine E in a case where only three cylinders operate in a no-load state or low-load state).

As long as such a catalyst preliminary heating operation is performed and the cut-off cylinder operation execution condition is fulfilled, the operations of steps ST1 to ST4 are repeated. Where the temperature of the first catalytic converter 82L becomes equal to or higher than the preliminary heating requiring temperature, while the cut-off cylinder operation execution condition is being fulfilled, it is determined NO in step ST3 and the processing flow advances to step ST6. In step ST6, the catalyst preliminary heating operation is stopped and the cut-off cylinder operation is executed when the temperature of the first catalytic converter 82L becomes equal to or higher than the preliminary heating requiring temperature.

In this case, as mentioned hereinabove, the torque of the engine E gradually decreases during the catalyst preliminary heating operation. Therefore, abrupt torque fluctuations during operation mode switching from the full-cylinder operation mode to the cut-off cylinder operation mode are prevented and the occurrence of vibrations (shocks) during the operation mode switching is practically eliminated. In other words, switching from the full-cylinder operation to the cut-off cylinder operation can be so conducted that the vehicle occupants will not be aware of the operation mode switching and drivability can be greatly improved.

As described hereinabove, in a case where the cut-off cylinder operation execution condition is not fulfilled, for example, when the engine load increases after the cut-off cylinder operation has been started, it is determined NO in step ST1, the processing flow advances to step ST7, and the full-cylinder operation is executed. In other words, when a state is assumed in which the cut-off cylinder operation execution condition is not fulfilled, the cut-off cylinder operation mode is switched to the full-cylinder operation mode.

In a state in which the cut-off cylinder operation is executed, it is determined NO in step ST2, the processing flow advances to step ST5, and it is determined whether the temperature of the first catalytic converter 82L is less than the predetermined full-cylinder operation restoration temperature (T2). In other words, it is determined whether the temperature of the first catalytic converter 82L has decreased to the vicinity of the lower limit of the activation temperature.

The temperature that is by about 50° C. higher than the lower limit of the activation temperature (for example, 450° C.) of the first catalytic converter 82L is set as the full-cylinder operation restoration temperature (T2). However, these values are not limiting.

When the temperature of the first catalytic converter 82L is equal to or higher than the full-cylinder operation restoration temperature (when it is determined NO in step ST5), the cut-off cylinder operation is continued as is.

Where the temperature of the first catalytic converter 82L is less than the full-cylinder operation restoration temperature and it is determined YES in step ST5, the processing flow advances to step ST7 and the full-cylinder operation is executed in order to avoid the degradation of exhaust emission during the full-cylinder operation restoration.

As described hereinabove, in the present embodiment, when the cut-off cylinder operation execution condition is fulfilled in the process of executing the full-cylinder operation and the temperature of the first catalytic converter 82L is less than the predetermined preliminary heating requiring temperature, first, the catalyst preliminary heating operation is executed to raise the temperature of the first catalytic converter 82L to a temperature equal to or higher than the preliminary heating requiring temperature, instead of starting the cut-off cylinder operation immediately after the cut-off cylinder operation execution conditions if fulfilled. The cut-off cylinder operation is then started after the temperature of the first catalytic converter 82L has been raised to a temperature equal to or higher than the preliminary heating requiring temperature by the execution of the catalyst preliminary heating operation. Therefore, because the cut-off cylinder operation is started in a state with a comparatively high temperature of the first catalytic converter 82L, even if the temperature of the first catalytic converter 82L decreases gradually during the cut-off cylinder operation, the time required for this temperature to decrease to a temperature at which the restoration of the full-cylinder operation is necessary (the full-cylinder operation restoration temperature) can be extended. As a result, the continuation time of the cut-off cylinder operation can be extended, merits of the engine system in which the cut-off cylinder operation can be executed can be used to a full extent, and the effect of improving the fuel consumption ratio and the effect of reducing the fuel consumption amount can be fully demonstrated.

In the present embodiment because the torque of the engine E decreases gradually during the catalyst preliminary heating operation, it is possible to realize a novel operation of switching from the full-cylinder operation to cut-off cylinder operation that causes practically no vibrations during the operation mode switching.

A variation example of the catalyst preliminary heating operation will be explained below. In the above-described embodiment, the operation of delaying the ignition timing of cylinders (cut-off scheduled cylinders) that are in an inoperative state during the cut-off cylinder operation is performed as the catalyst preliminary heating operation. However, executing the below-described catalyst preliminary heating operation instead will also be within the technical scope of the invention.

First, no ignition of the spark plug 77L is performed with respect to the cylinders (cut-off scheduled cylinders) that will be in an inoperative state during the cut-off cylinder operation. In other words, during the catalyst preliminary heating operation, only fuel supply is performed with respect to the cut-off scheduled cylinders and no ignition of the spark plug 77L is performed.

As a result, a major portion of the air-fuel mixture contained in the cylinder enters as an unburned gas the exhaust pipe 81L, thermal energy of the first catalytic converter 82L is received by the mixture, and the mixture is burned (oxidation reaction). In this case, similarly to the catalyst preliminary heating operation of the aforedescribed embodiment, the temperature of the first catalytic converter 82L also can be raised within a short time to a temperature that is equal to or higher than the preliminary heating requiring temperature. Further, in this case, almost the entire amount of fuel supplied within the catalyst preliminary heating operation period can be caused to participate in the temperature increase of the first catalytic converter 82L. As a result, the amount of fuel used for the catalyst preliminary heating operation can be reduced to a necessary minimum.

Thus, where the catalyst preliminary heating operation is executed that un-ignites the spark plug 77L, certain thermal energy has to be present inside the first catalytic converter 82L. Therefore, this operation is performed after the temperature of the first catalytic converter 82L has been detected or estimated and the presence of thermal energy sufficient for burning the air-fuel mixture inside the first catalytic converter 82L has been confirmed. In other words, the temperature at which the air-fuel mixture can burn inside the first catalytic converter 82L is set as the lower limit of the activation temperature (T3).

As another variation example of the catalyst preliminary heating operation, when the engine E is a gasoline engine of a direct injection type, the fuel injection timing is controlled from the injector 75L that supplies the fuel to the cylinders (cut-off scheduled cylinders) that are in an inoperative state in the cut-off cylinder operation.

In other words, the fuel injection timing from this injector 75L is delayed. For example, in a case where the fuel injection timing is delayed to a timing about equal to the ignition timing of the spark plug 77L, the combustion start timing moves to the delay side because the delay is in the cut-off scheduled cylinders. Further, when the fuel injection timing is delayed more than the ignition timing of the spark plug 77L, no combustion is conducted inside the cylinders. In either case, the air-fuel mixture burns in the exhaust pipe 81L and the first catalytic converter 82L. Therefore, the temperature of the first catalytic converter 82L can be rapidly increased and the effect similar to that of the above-described embodiment can be demonstrated.

After the catalyst preliminary heating operation has been started, the delay amount of the fuel injection timing may be gradually increased. For example, the ignition timing is delayed by 1° CA (1° of crank angle) for every 10 revolutions of the crankshaft C. However, this value is not limiting and can be determined in advance by a test or simulation.

In the above-described embodiments, a case is explained in which the invention is applied to the V-type engine E for an automobile. However, the invention is not limited to this application and can be also applied to a horizontal opposing engine for an automobile and a linear engine for an automobile. Further, the invention can be applied not only to a gasoline engine, but also do a diesel engine. In a case of application to a diesel engine, because engines of this type are typically not provided with spark plugs, the catalyst preliminary heating operation is executed by controlling the fuel injection timing from the injector to the delay side. Further, the invention is nor particularly limited to the number of cylinders in the engine, fuel injection system, and also specifications of the engine E.

In the above-described embodiments, the ignition timing of the cut-off scheduled cylinders is gradually delayed as the catalyst preliminary heating operation. However, the invention is not limited to this configuration and it is also possible to move the ignition timing of the cut-off scheduled cylinders significantly to the delay side simultaneously with the start of the catalyst preliminary heating operation and maintain such ignition timing (maintain till the temperature of the first catalytic converter 82L reaches the preliminary heating requiring temperature).

The invention can be applied to a control conducted during switching from a full-cylinder operation mode to a cut-off cylinder operation mode in an automobile engine in which the cut-off cylinder operation can be executed.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention. 

1. An operation control device for a multi-cylinder internal combustion engine which is provided with a first catalyst provided in a first exhaust passage linked to at least one cylinder from among a plurality of cylinders and a second catalyst provided in a second exhaust passage linked to other cylinders, and in which the at least one cylinder is made inoperative by cut-off cylinder operation when a predetermined cut-off cylinder operation execution condition is fulfilled, comprising: a catalyst temperature recognition portion that detects or executes a temperature of the first catalyst; and a catalyst preliminary heating operation execution portion that executes a catalyst preliminary heating operation for increasing a temperature of the first catalyst to a temperature equal to or higher than a preliminary heating requiring temperature, which is higher than a lower limit of activation temperature of the first catalytic converter by a predetermined amount, before the cut-off cylinder operation is executed in a case where the temperature of the first catalyst that has been detected or estimated by the catalyst temperature recognition portion is less than the predetermined preliminary heating requiring temperature when the cut-off cylinder operation execution condition is fulfilled, wherein the catalyst preliminary heating operation delays an ignition timing of a spark plug of the at least one cylinder that becomes inoperative during the cut-off cylinder operation prior to executing the cut-off cylinder operation and gradually increases a delay amount of the ignition timing, thereby gradually decreasing an amount of kinetic energy participating in torque generation in the internal combustion engine and gradually increasing an amount of thermal energy participating in the catalyst preliminary heating, from among an energy generated by combustion of an air-fuel mixture, and wherein, while the cut-off cylinder operation execution condition is being fulfilled, when the temperature of the first catalyst becomes equal to or higher than the preliminary heating requiring temperature due to the catalyst preliminary heating operation, the catalyst preliminary heating operation is stopped and the cut-off cylinder operation is started. 2-4. (canceled)
 5. The operation control device for a multi-cylinder internal combustion engine according to claim 1, wherein in a case where the temperature of the first catalyst has decreased to a predetermined full-cylinder operation restoration temperature during the cut-off cylinder operation, switching is made from the cut-off cylinder operation to the full-cylinder operation, and the full-cylinder operation restoration temperature is set to a temperature that is higher than the lower limit of activation temperature of the first catalyst at which the air-fuel mixture can burn inside the first catalyst and to a temperature that is lower than the preliminary heating requiring temperature.
 6. The operation control device for a multi-cylinder internal combustion engine according to claim 1, wherein the predetermined amount is 150° C.
 7. An operation control method for a multi-cylinder internal combustion engine which is provided with a first catalyst provided in a first exhaust passage linked to at least one cylinder from among a plurality of cylinders and a second catalyst provided in a second exhaust passage linked to other cylinders, and in which the at least one cylinder is made inoperative by cut-off cylinder operation, the method comprising: determining whether a predetermined cut-off cylinder operation execution condition is fulfilled; detecting or estimating a temperature of the first catalyst; executing a catalyst preliminary heating operation for increasing a temperature of the first catalyst to a temperature equal to or higher than a preliminary heating requiring temperature, which is higher than a lower limit of activation temperature of the first catalytic converter by a predetermined amount, before the cut-off cylinder operation is executed in a case where the temperature of the first catalyst that has been detected or estimated is less than the predetermined preliminary heating requiring temperature when the cut-off cylinder operation execution condition is fulfilled, wherein the catalyst preliminary heating operation delays an ignition timing of a spark plug of the at least one cylinder that becomes imperative during the cut-off cylinder operation prior to executing the cut-off cylinder operation and gradually increases a delay amount of the ignition timing, thereby gradually decreasing an amount of kinetic energy participating in torque generation in the internal combustion engine gradually increasing an mount of thermal energy participating in the catalyst preliminary heating, from among an energy generated by combustion of an air-fuel mixture; and ceasing the catalyst preliminary heating operation and starting the cut-off cylinder operation, when the temperature of the first catalyst becomes equal to or higher than the preliminary heating requiring temperature due to the catalyst preliminary heating operation, while the cut-off cylinder operation execution condition is being fulfilled. 